SQL is an indispensable relational database query statement. The DISTINCT keyword serves as a prevalent and potent tool in retrieving unique values from a database. In this article, we'll delve into a thorough examination of the SQL DISTINCT statement, its syntax, implementation scenarios, and hands-on examples.

What Exactly is SQL DISTINCT?

SQL DISTINCT is a clause that can be employed within SELECT statements to eradicate duplicate values and produce only unique values or records. It functions on a per-column basis, meaning that if one desires to eliminate duplicates based on multiple columns, the utilization of the GROUP BY clause is required.

  1. Overview: The DISTINCT clause, when utilized within a SELECT statement, serves the purpose of eliminating duplicative rows from the result set of a query. The resulting output consists of unique values only, effectively streamlining the data for further analysis or manipulation.

  2. Syntax: The basic syntax for employing the DISTINCT clause follows the format: SELECT DISTINCT column_name FROM table_name. In the event that multiple columns require distinct values, the list of column names can be extended within the query.

  3. Aggregates: The statement can be used with aggregates: COUNT, AVG, MAX, etc

  4. Utilization Scenarios: Common utilization scenarios for the DISTINCT clause include, but are not limited to, the removal of duplicates, counting of unique values, data aggregation, and the generation of a list of unique values.

  5. Advantages: The advantages of utilizing the DISTINCT clause include saving time and effort, reduction of data set size, and improvement in the meaningfulness and usefulness of the data.

  6. Limitations: It is important to note that the DISTINCT clause operates on a single-column basis, thus requiring a combination of the DISTINCT clause and the GROUP BY clause to eliminate duplicates across multiple columns.

  7. Performance Considerations: The performance implications of the DISTINCT clause are contingent upon the size of the data set and the complexity of the query in question. As such, it is advisable to optimize queries for performance and scalability.

  8. Best Practices: Best practices in utilizing the DISTINCT clause encompass the utilization of indexes, avoidance of subqueries, and maintenance of a minimal data set size.

There may be a situation when you have multiple duplicate records in a table. while fetching such records, it makes more sense to fetch only those unique records instead of fetching duplicate records
Inside a table, a column often contains many duplicate values and sometimes you only want to list the different (distinct) values
Syntax
  1. SELECT DISTINCT Emp_Name
  2. FROM EmployeeDetail;
Example

SELECT COUNT DISTINCT statement

The SELECT DISTINCT COUNT() function returns the number of rows that matches a specified criteria
The following SQL statement lists the number of different (DISTINCT)EmployeeDetails EmpId
Syntax
  1. SELECT COUNT(DISTINCT EmpId) FROM EmpDetail;
Example

SELECT AVG DISTINCT statement

The AVG DISTINCT () function returns the average value of a numeric column
The following SQL statement lists the number of different (DISTINCT)EmployeeDetails EmpId
Syntax
  1. SELECT AVG(DISTINCT EmpId)
  2. FROM EmployeeDetail
  3. WHERE EmpId= EmpId
Example
DISTRICT

SELECT MAX DISTINCT statement

The SELECT MAX DISTINCT function returns the largest value of the selected column
Syntax
  1. SELECT MAX(DISTINCT EmpId)
  2. FROM EmployeeDetail
  3. WHERE EmpId= EmpId
Example
NOTE
This example above will not work in Firefox and Microsoft edge! Because COUNT(DISTINCT column_name) is not supported in Microsoft access databases firefox and
Microsoft Edge is using Microsoft access in our examples
Here is the workaround for ms access

SQL SELECT DISTINCT on multiple columns

Here is a simple Example on some selected columns in EmpoyeeDetail table where EmpId= 5
Syntax
  1. SELECT DISTINCT EmpName,EmpAddress,EmpCity FROM EmployeeDetail Where EmpId=5
Example
SQL SELECT DISTRICT

SELECT example without DISTINCT

The following SQL statement selects all (including the duplicates) values from the EmpName column in the EmployeeDetail table
Syntax
  1. SELECT * FROM EmployeeDetail
Example

Summary

That's all there is to it, folks! Just remember, whether you're removing duplicates, aggregating data, counting unique values, or creating a unique list of values, the DISTINCT clause is a valuable tool that'll make your work easier and faster.